A hydraulic system was designed and sold by a new sales engineer. The application required an ejector cylinder to remain fully retracted for a dwell period while the molded part cured. Once the part was fully cured, the cylinder would extend to eject the part, emptying the mold for the next cycle. The circuit used in the design is shown below.

After the system was installed, the customer complained that every time the machine dwelled more than six to 10 sec, the ejector cylinder began to drift out, extending into the mold cavity. This caused unwanted recesses in the cured part. The customer overcame this problem by leaving the retract solenoid energized during the long curing process.

The customer’s maintenance department installed pilot-operated check valves in the A and B ports, thinking this would solve the problem. After air was bled from the system, it started to drift again. Thinking he had a bad cylinder, the maintenance supervisor ordered a new cylinder. But, again, once the air was bleed from the system, the new cylinder started to drift, just as before.